The present invention relates to a system for the generation of a time variant signal for suppression of a primary signal, comprising:
a control unit at least provided with one digital filter, an input for receiving an update signal for updating coefficients of the digital filter and an output for providing a cancellation control signal; PA1 cancellation-generating means which are connected to the output of the control unit for the generation of a cancellation signal, which is intended, after propagation along a secondary transfer path having a path transfer function, to be added as the time variant signal at an addition point to the primary signal in order to provide a residual signal, PA1 sensor means for measuring the residual signal at the addition point and for providing an output signal; PA1 update means provided with an input which is connected to the sensor means and an output for providing the update signal. PA1 disturbances in the residual signal which are not part of the reference signal, for example an alinear relationship between the reference signal and the output signal from the sensor means, PA1 disturbances in the residual signal which arise only subsequently in the reference signal, such as can easily be the case when vibrations are cancelled out, PA1 changes in the acoustic path between cancellation control signal and residual signal, for example as a consequence of a change in temperature. PA1 W indicates a first time vector EQU W(t)=[w.sub.0 (t) w.sub.1 (t) . . . w.sub.nw (t)] PA1 R indicates a second time vector EQU R(t)=[1 r.sub.1 (t) . . . r.sub.nr (t)] PA1 S indicates a third time vector EQU S(t)=[s.sub.0 (t) s.sub.1 (t) . . . s.sub.ns (t)] PA1 .mu.(t)=step size parameter PA1 F.sup.-1 =a matrix for optimising the direction. EQU .theta.=[1r.sub.1 (t) . . . r.sub.nr (t)/w.sub.o (t) . . . w.sub.nw (t)/ s.sub.o (t) . . . s.sub.ns (t)] PA1 F.sup.-1 (t)=direction optimalisation matrix ##EQU3## and the control unit is equipped to update the filter coefficients of the forward filter having transfer function --W/R and the feedback filter having transfer function --S/R in accordance with: ##EQU4##
A system of this type is disclosed in US Patent 4,677,676, in which a system for the generation of an estimated time variant signal is described which, for example, can be used in the field of noise or vibration suppression. The known system has to generate a cancellation signal which has an amplitude which is at least approximately of equal magnitude but of opposite sign to a primary signal, so that the effect of the primary signal can be cancelled by adding the two signals.
The known system comprises a control unit which is connected to a sensor which detects the primary signal and a sensor which detects a residual signal, that is to say the signal which remains after adding the primary signal and the generated cancellation signal. The coefficients of the digital filter can be adapted by the residual signal.
The convergence speed and stability of the known system are adversely affected by the time delay and the possible phase shift between the output of the control unit and the location where the cancellation signal is added to the primary signal in order as far as possible to cancel the primary signal. In an anti-noise system, for example, the output signal from the control unit is converted between the output of the control unit and the addition point into an acoustic signal, which traverses an acoustic path. The path is indeed termed the secondary acoustic path, in contrast to the primary acoustic path, which is traversed by the primary signal itself. The delays associated with acoustic paths are appreciable compared with the delays to which electrical signals are subject. In the known system no account is taken of the influence of the transfer function associated with the acoustic path, which has an adverse effect on the convergence of the calculations in the filter in the control unit. The same applies in the case of vibration systems, in which undesirable vibrations are propagated by a mechanical construction and have to be cancelled out with the aid of a vibration generator, anti-vibrations generated being propagated by a secondary vibration path.